The term “zeolite” was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that had previously been adsorbed into the material. Based on this, the called the material zeolite, from the Greek zeo, meaning “boil” and lithos, meaning “stone”.
We now know that zeolites are microporous, aluminosilicate or silicate minerals. As of November 2010, 194 unique zeolite frameworks have been identified (DDR being one of them), and over 40 naturally occurring zeolite frameworks are known.
Zeolites have a porous structure that can accommodate a wide variety of cations, such as Na+, K+, Ca2+, Mg2+ and many others. These positive ions are rather loosely held and can readily be exchanged for others in a contact solution. Some of the more common mineral zeolites are analcime, chabazite, clinoptilolite, heulandite, natrolite, phillipsite, and stilbite.
The regular pore structure and the ability to vary pore size, shape and chemical nature makes zeolites very useful as molecular sieves. Depending on their structure and composition, zeolites can separate molecules based on adsorption and/or diffusion of certain molecules preferentially inside the pores or exclusion of certain molecules based on their size. The pore size is typically less than 2 nm and comparable to that of small molecules, allowing the use of zeolites to separate lightweight gases such as CO2 and CH4.
The maximum size of a species that can enter the pores of a zeolite is controlled by the dimensions of the channels in the zeolite. These are conventionally defined by the ring size of the aperture, where, for example, the term “8-ring” refers to a closed loop that is built from 8 tetrahedrally coordinated silicon (or aluminum) atoms and 8 oxygen atoms. The rings are not always symmetrical due to a variety of effects, including strain induced by the bonding between units that are needed to produce the overall structure, or coordination of some of the oxygen atoms of the rings to cations within the structure. Therefore, the pores in many zeolites are not cylindrical. The DDR zeolite of this invention has an 8-ring structure (see FIG. 1).
Thus, zeolites are widely used in industry for water purification, as catalysts, and in nuclear reprocessing. Their biggest use is in the production of laundry detergents, and they are also used in medicine and in agriculture.
In particular, zeolites have been used in two types of molecular sieving membranes: mixed matrix membranes and pure zeolite membranes. To fabricate a mixed matrix membranes, zeolite crystals are first dispersed in a polymer solution. The dispersion is then cast into a film or spun into a tubular hollow fiber. Since the membrane thickness is desired to be less than 1 micron, it is necessary to have submicron zeolite particles.
In pure zeolite membrane fabrication, zeolite crystals are first deposited as a “seed” coating on a porous substrate and then grown into a thin continuous layer known as a zeolite membrane. The porous substrate provides mechanical stability for the membrane. In this approach crystals with submicron size are also preferred because the seed coatings will then be tightly packed and of high quality. Further, membrane thickness is ideally about 0.5-5 microns.
Among the various zeolite materials, DDR is a pure silica (SiO2) zeolite. The dimensions of the molecular sieving pores of the DDR zeolite are 3.6×4.4 angstrom. Due to its relatively small pore size, DDR can be used to separate light gases, such as CO2 (kinetic diameter=3.3 angstroms) from CH4 (diameter=3.8 angstroms). Other advantages of DDR zeolites include high thermal stability and chemical resistance due to the pure silica composition.
DDR zeolite crystals were first synthesized in 19861 and the synthesis was further developed by several researchers2-4. These synthesis methods either take long time (9-25 days) or produce very large crystals (20 to 50 micrometers). DDR zeolite membranes were first reported in 20045 and the results showed that CO2/CH4 selectivity was 220 at 301° K with feed pressure of 0.5 MPa.
However, there is no prior art on the synthesis of micron or nanometer sized DDR crystals, which are critical in fabricating high-quality membranes. In this disclosure, methods for synthesizing nanometer to micron size DDR zeolite crystals are described. Not only can the size and shape of the DDR crystal be controlled, but the synthesis time is significantly shortened. Thus, the methods and compositions described herein are a significant improvement on the prior art.